COMBUSTION AND FLAME

OMBUSTION AND FLAME

e use differ

ent kinds of fuel for

various purposes at home, in

industry and for running

automobiles. Can you name a few fuels

used in our homes? Name a few fuels

used in trade and industry. What fuels

e used for running automobiles? Your

list will contain fuels like cowdung,

wood, coal, charcoal, petrol, diesel,

compressed natural gas (CNG), etc.

You are familiar with the burning of a

candle. What is the difference between the

burning of a candle and the burning of a

fuel like coal? May be you were able to

guess right: candle burns with a flame

whereas coal does not. Similarly, you will

find many other materials burning

without a flame. Let us study the chemical

process of burning and the types of flame

produced during this process.

4.1 What is Combustion?

Recall the activity of burning of

magnesium ribbon performed in

Class VII. We learnt that magnesium

burns to form magnesium oxide and

produces heat and light (Fig. 4.1).

We can perform a similar activity with

a piece of charcoal. Hold the piece with

a pair of tongs and bring it near the

flame of a candle or a Bunsen burner.

What do you observe?

We find that charcoal burns in air.

We know that coal, too, burns in air

producing carbon dioxide, heat and

light.

We were told that

food is a fuel for

our body.

Fig. 4.1 : Burning of magnesium

A chemical process in which a

substance reacts with oxygen to give

off heat is called combustion. The

substance that undergoes combustion

is said to be combustible. It is also called

a fuel. The fuel may be solid, liquid or

gas. Sometimes, light is also given off

during combustion, either as a flame or

as a glow.

In the reactions mentioned

above magnesium and charcoal are

combustible substances.

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Activity 4.2

(Caution : Be careful while handling

burning candle).

Fix a lighted candle on a table. Put

a glass chimney over the candle and

rest it on a few wooden blocks in

such a way that air can enter the

Table 4.1 : Combustible and Non-

combustible Substances

Material Combustible Non-

combustible

Wood

Paper

Iron nails

Kerosene oil

Stone piece

Straw

Charcoal

Matchsticks

Glass

Activity 4.1

Collect some materials like straw,

matchsticks, kerosene oil, paper,

iron nails, stone pieces, glass etc.

Under the supervision of your

teacher

try to burn each of these

materials one by one. If combustion

takes place mark the material

combustible, otherwise mark it

non-combustible

(Table 4.1).

(a) (b) (c)

Fig. 4.2: Experiment to show that air is

essential for burning

Rightly so. In our body

food is broken down by

reaction with oxygen and

heat is produced. We

learnt that in Class VII.

Can you name some more

substances which are combustible? You

can add those to Table 4.1.

Let us investigate conditions under

which combustion takes place.

COMBUSTION AND FLAME

chimney [Fig. 4.2(a)]. Observe what

happens to the flame. Now remove

the blocks and let the chimney rest

on the table [Fig. 4.2(b)]. Again

observe the flame. Finally, put a

glass plate over the chimney [Fig.

4.2(c)]. Watch the flame again. What

happens in the three cases? Does

the flame flicker off? Does it flicker

and give smoke? Does it burn

unaffected? Can you infer anything

at all about the role played by air in

the process of burning?

We find that for combustion, air is

necessary. The candle burns freely in

case (a) when air can enter the

chimney from below. In case (b), when

air does not enter the chimney from

below, the flame flickers and produces

smoke. In case (c), the flame finally

goes off because the air is not

available.

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Activity 4.3

Place a piece of burning wood or

charcoal on an iron plate or Tawa.

Cover it with a glass jar or a

tumbler, or a transpar

ent plastic jar.

Observe what happens. Does

charcoal stop burning after

sometime? Can you think of the

reason why it stops burning?

You might have heard that when the

clothes of a person catch fire, the person

We have r

ead that the sun

produces its own heat and

light. Is it also some kind of

combustion?

In the sun, heat and light are

produced by nuclear reactions. You

will learn about this process in higher

classes.

During extreme heat

of summer, at some

places dry grass

catches fire. From

the grass, it spreads

to trees, and very

soon the whole forest

is on fire (Fig. 4.4). It

is very difficult to

control such fires.

Fig. 4.4 : Forest fire

Fig. 4.3 : Blanket wrapped around a person

whose clothes caught fire

Now recall some of your experiences.

Does a matchstick burn by itself?

How does it burn?

You must have had an experience of

burning a piece of paper. Does it burn

when a burning matchstick is brought

near it?

Can you burn a piece of wood by

bringing a lighted matchstick near it?

Why do you have to use paper or

kerosene oil to start fire in wood or coal?

Have you heard of forest fires?

is covered with a blanket to extinguish

fire (Fig. 4.3). Can you guess why?

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COMBUSTION AND FLAME

We find that a combustible substance

cannot catch fire or burn as long as its

temperature is lower than its ignition

temperature. Have you ever seen

cooking oil catching fire when a frying

pan is kept for long on a burning stove?

Kerosene oil and wood do not catch fire

on their own at room temperature. But,

if kerosene oil is heated a little, it will

catch fire. But if wood is heated a little,

it would still not catch fire. Does it mean

that ignition temperature of kerosene oil

is lower than that of wood? Does it mean

that we need to take special care in

storing kerosene oil? The following

activity shows that it is essential for a

substance to reach ignition temperature

to burn.

Activity 4.4

(Caution : Be careful while handling

burning candle).

Make two paper cups by folding a

sheet of paper. Pour about 50 mL of

water in one of the cups. Heat both

the cups separately with a candle

(Fig. 4.5). What do you observe?

The history of the matchstick is very

old. More than five thousand years

ago small pieces of pinewood dipped

in sulphur were used as matches in

ancient Egypt. The modern safety

match was developed only about two

hundred years ago.

A mixture of antimony trisulphide,

potassium chlorate and white

phosphorus with some glue and

starch was applied on the head of a

match made of suitable wood. When

struck against a rough surface, white

phosphorus got ignited due to the heat

of friction. This started the

combustion of the match. However,

white phosphorus proved to be

dangerous both for the workers

involved in the manufacturing of

matches and for the users.

These days the head of the safety

match contains only antimony

trisulphide and potassium chlorate.

The rubbing surface has powdered

glass and a little red phosphorus

(which is much less dangerous).

When the match is struck against

the rubbing surface, some red

phosphorus gets converted into white

phosphorus. This immediately reacts

with potassium chlorate in the

matchstick head to produce enough

heat to ignite antimony trisulphide

and start the combustion.

Do these experiences tell you that

different substances catch fire at

different temperatures?

The lowest temperature at which a

substance catches fire is called its

ignition temperature.

Can you tell now why a matchstick

does not catch fire on its own at room

temperature? Why does the matchstick

start burning on rubbing it on the side

of the matchbox?

Fig. 4.5 : Heating water in a paper cup

water

paper

cups

What happens to the empty paper

cup? What happens to the paper cup

with water? Does water in this cup

become hot?

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If we continue heating

the cup, we can even boil

water in the paper cup.

Can you think of an

explanation for this

phenomenon?

The heat supplied to

the paper cup is

transferred to water by

conduction. So, in the

presence of water, the

ignition temperature of

paper is not reached.

Hence, it does not burn.

The substances which

have very low ignition

temperature and can

easily catch fire with

a flame are called

inflammable substances. Examples of

inflammable substances are petrol,

alcohol, Liquified Petroleum Gas (LPG)

etc. Can you list some more inflammable

substances?

4.2 How Do We Control Fire?

You must have seen or heard of fire

breaking out in homes, shops and

factories. If you have seen such an

accident, write a short description in

your note book. Also, share the

experience with your classmates.

It is important that all of us

know the telephone numbers

of the fire service.

Fig. 4.6: Firemen extinguish the fire by throwing water under pressure

Find out the telephone number of the

fire service in your area. If a fire

breaks out in your house or in your

neighbourhood, the first thing to do

is to call the fire service.

Does your city/town have a fire

brigade station?

When a fire brigade arrives, what

does it do? It pours water on the fire

(Fig. 4.6). Water cools the combustible

material so that its temperature is

brought below its ignition

temperature. This prevents the fire

from spreading. Water vapours also

surround the combustible material,

helping in cutting off the supply of air.

So, the fire is extinguished.

You have lear

nt that there are three

essential requirements for producing

fire. Can you list these requirements?

These are: fuel, air (to supply

oxygen) and heat (to raise the

temperature of the fuel beyond the

ignition temperature). Fire can be

controlled by removing one or more of

these requirements. The job of a fire

extinguisher is to cut off the supply of

air, or to bring down the temperature

of the fuel, or both. Notice that the fuel

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COMBUSTION AND FLAME

The most common fire

extinguisher is water. But

water works only when

things like wood and paper

are on fire. If electrical

equipment is on fire, water

may conduct electricity

and harm those trying to

douse the fire. Water is

also not suitable for fires

involving oil and petrol. Do

you recall that water is

heavier than oil? So, it

sinks below the oil, and oil

keeps burning on the top.

For fires involving electrical equipment and inflammable materials like

petrol, carbon dioxide (CO

) is the best extinguisher. CO

, being heavier than

oxygen, covers the fire like a blanket. Since the contact between the fuel and

oxygen is cut off, the fire is controlled. The added advantage of CO

is that in

most cases it does not harm the electrical equipment.

How do we get the supply of carbon dioxide? It can be stored at high pressure

as a liquid in cylinders. In what form is the LPG stored in cylinders? When

released from the cylinder, CO

expands enormously in volume and cools down.

So, it not only forms a blanket around the fire, it also brings down the

temperature of the fuel. That is why it is an excellent fire extinguisher. Another

way to get CO

is to release a lot of dry powder of chemicals like sodium

bicarbonate (baking soda) or potassium bicarbonate. Near the fire, these

chemicals give off CO

CAUTION : Do not handle the gas

stove yourself. Ask your parents

to help.

Fig. 4.7 : Fire extinguisher

in most cases cannot be eliminated.

If, for instance, a building catches fire,

the whole building is the fuel.

4.3 Types of Combustion

Bring a burning matchstick or a gas

lighter near a gas stove in the kitchen.

Turn on the knob of the gas stove. What

do you observe?

We find that the gas burns rapidly

and produces heat and light. Such

combustion is known as rapid

combustion.

There are substances like

phosphorus which burn in air at room

temperature.

The type of combustion in which

a material suddenly bursts into

flames, without the application of any

apparent cause is called

spontaneous

combustion.

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Spontaneous combustion of coal dust

has resulted in many disastrous fires

in coal mines. Spontaneous forest

fires are sometimes due to the heat

of the sun or due to lightning strike.

However, most forest fires are due to

the carelessness of human beings. It

is important to remember that the

campfires must be completely

extinguished before leaving a forest

after a picnic, or a visit.

We generally have fireworks on

festival days. When a cracker is ignited,

a sudden reaction takes place with the

evolution of heat, light and sound. A

large amount of gas formed in the

reaction is liberated. Such a reaction is

called explosion. Explosion can also take

place if pressure is applied on the

cracker.

4.4 Flame

Observe an LPG flame. Can you tell the

colour of the flame. What is the colour

of a candle flame?

Recall your experience of burning a

magnesium ribbon in Class VII. If you

do not have experience of burning the

remaining items in Table 4.2 you can

do that now.

Table 4.2 Materials forming Flame on Burning

S.No. Material Forms flame Does not form flame

1. Candle

2. Magnesium

3. Camphor

4. Kerosene Stove

5. Charcoal

Fig. 4.9 : Flames of kerosene lamp, candle and

Bunsen burner

Fig. 4.8: Colours of a candle flame and the

flame of a kitchen stove

Record your observations and

mention whether on burning the

material forms a flame or not.

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COMBUSTION AND FLAME

4.5 Structure of a Flame

Activity 4.5

Light a candle (Caution : Be careful).

Hold a 4

5 cm long thin glass tube

with a pair of tongs and introduce

its one end in the dark zone of a

non-flickering candle flame

(Fig. 4.10). Bring a lighted

matchstick near the other end of the

glass tube. Do you see a flame

caught at this end of the glass tube

after a while? If so, what is it that

produces a flame? Notice that the wax

near the heated wick melts quickly.

The substances which vapourise

during burning, give flames. For

example, kerosene oil and molten wax

rise through the wick and are vapourised

during burning and form flames.

Charcoal, on the other hand, does not

vapourise and so does not produce a

flame. In Activity 4.5, could the vapours

of wax coming out of the glass tube be

the cause of the flame produced?

Fig. 4.10

Fig. 4.12

When the candle flame is steady,

introduce a clean glass plate/slide into

the luminous zone of the flame

(Fig. 4.11). Hold it there with a pair of

tongs for about 10 seconds. Then

remove it. What do you observe?

Fig. 4.11

A circular blackish ring is formed on

the glass plate/slide. It indicates the

deposition of unburnt carbon particles

present in the luminous zone of the

flame.

Hold a thin long copper wire just

inside the non-luminous zone of flame

for about 30 seconds (Fig. 4.12).

Notice that the portion of the copper

wire just outside the flame gets red hot.

Does it indicate that the non-luminous

zone of the flame has a high

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SCIENCE48

Fig. 4.14 : Goldsmith blowing through a

metallic pipe

4.6 What is a Fuel?

Recall that the sources of heat energy

for domestic and industrial purposes

are mainly wood, charcoal, petrol,

kerosene etc. These substances are

called fuels. A good fuel is one which is

readily available. It is cheap. It burns

easily in air at a moderate rate. It

produces a large amount of heat. It does

not leave behind any undersirable

substances.

There is probably no fuel that could

be considered as an ideal fuel. We

should look for a fuel which fulfils most

of the requirements for a particular use.

Fuels differ in their cost. Some fuels

are cheaper than others.

Make a list of fuels familiar to you.

Group them as solid, liquid and gaseous

fuels as in Table 4.3.

4.7 Fuel Efficiency

Suppose you were asked to boil a given

quantity of water using cow dung, coal

and LPG as fuel. Which fuel would you

prefer? Give your r

eason. You may take

the help of your parents. Do these three

fuels produce the same amount of heat?

The amount of heat energy produced on

complete combustion of 1 kg of a fuel is

called its calorific value. The calorific

value of a fuel is expressed in a unit

Fig. 4.13 : Different zones of candle flame

Table 4.3 : Types of Fuels

S. No. Solid Fuels Liquid Fuels Gaseous Fuels

1. Coal Kerosene oil Natural gas

hottest

part

moderately

hot

least

hot

wax candle

outer zone of

complete

combustion (blue)

middle zone of

partial combustion

(yellow)

innermost zone of

unburnt wax

vapours (black)

temperature? In fact, this part of the

flame is the hottest part (Fig. 4.13).

Goldsmiths blow the outermost zone

of a flame with a metallic blow-pipe for

melting gold and silver (Fig. 4.14). Why

do they use the outermost zone of the

flame?

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COMBUSTION AND FLAME

particles. These fine particles are

dangerous pollutants causing

respiratory diseases, such as asthma.

2. Incomplete combustion of these

fuels gives carbon monoxide gas. It is a

very poisonous gas. It is dangerous to

burn coal in a closed room. The carbon

monoxide gas produced can kill persons

sleeping in that room.

For centuries, wood was used as

domestic and industrial fuel. But now

it has been replaced by coal and other

fuels like LPG. In many rural parts of

our country, people still use wood as a

fuel because of its easy availability and

low cost. However, burning of wood gives

a lot of smoke which is very harmful for

human beings. It causes respiratory

problem. Also, trees provide us with

useful substances which are lost when

wood is used as fuel. Moreover cutting

of trees leads to deforestation which is

quite harmful to the environment, as

you learnt in Class VII.

Table 4.4 : Calorific Values of different

Fuels

Fuel Calorific Value

(kJ/kg)

Cow dung cake 6000-8000

Wood 17000-22000

Coal 25000-33000

Petrol 45000

Kerosene 45000

Diesel 45000

Methane 50000

CNG 50000

LPG 55000

Biogas 35000-40000

Hydrogen 150000

Oh! So, that is why we are

advised never to sleep in a

room with burning or

smouldering coal fire in it.

Burning of Fuels Leads to Harmful

Products

The increasing fuel consumption has

harmful effects on the environment.

1. Carbon fuels like wood, coal,

petroleum release unburnt carbon

3. Combustion of most fuels releases

carbon dioxide in the environment.

Increased concentration of carbon

dioxide in the air is believed to cause

global warming.

Global warming is the rise in

temperature of the atmosphere of the

earth. This results, among other things,

in the melting of polar glaciers, which

leads to a rise in the sea level, causing

floods in the coastal areas. Low lying

coastal areas may even be permanently

submerged under water.

4. Burning of coal and diesel releases

sulphur dioxide gas. It is an extremely

suffocating and corrosive gas. Moreover,

petrol engines give off gaseous oxides

of nitrogen. Oxides of sulphur and

nitrogen dissolve in rain water and form

acids. Such rain is called acid rain. It is

very harmful for crops, buildings

and soil.

The use of diesel and petrol as fuels

in automobiles is being replaced by

CNG (Compressed Natural Gas), because

CNG produces the harmful products in

very small amounts. CNG is a cleaner fuel.

called kilojoule per kg (kJ/kg).

Calorific values of some fuels are given

in Table 4.4.

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KEYWORDS KEYWORDS

KEYWORDS

ACID RAIN

CALORIFIC VALUE

COMBUSTION

DEFORESTATION

EXPLOSION

FLAME

FIRE EXTINGUISHER

FUEL

FUEL EFFICIENCY

GLOBAL WARMING

IDEAL FUEL

IGNITION

TEMPERATURE

INFLAMMABLE

SUBSTANCES

WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT

WHAT YOU HAVE LEARNT

Ü The substances which burn in air are called

combustible.

Ü Oxygen (in air) is essential for combustion.

Ü During the process of combustion, heat and

light are given out.

Ü Ignition temperature is the lowest temperature

at which a combustible substance catches

fire.

Ü Inflammable substances have very low

ignition temperature.

Ü Fire can be controlled by removing one or more

requirements essential for producing fire.

Ü Water is commonly used to control fires.

Ü Water cannot be used to control fires involving

electrical equipment or oils.

Ü There are various types of combustions such

as rapid combustion, spontaneous

combustion, explosion, etc.

Ü There are three different zones of a flame -

dark zone, luminous zone and non-luminous

zone.

Ü An ideal fuel is cheap, readily available, readily

combustible and easy to transport. It has high

calorific value. It does not produce gases or

residues that pollute the environment.

Ü Fuels differ in their efficiency and cost.

Ü Fuel efficiency is expressed in terms of its

calorific value which is expressed in units of

kilojoule per kg.

Ü Unburnt carbon particles in air are dangerous

pollutants causing respiratory problems.

Ü Incomplete combustion of a fuel gives

poisonous carbon monoxide gas.

Ü Increased percentage of carbon dioxide in air

has been linked to global warming.

Ü Oxides of sulphur and nitrogen produced by

the burning of coal, diesel and petrol cause

acid rain which is harmful for crops, buildings

and soil.

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COMBUSTION AND FLAME

Exercises

1. List conditions under which combustion can take place.

2. Fill in the blanks.

(a) Burning of wood and coal causes of air.

(b) A liquid fuel, used in homes is

before it starts

burning.

(d) Fire produced by oil cannot be controlled by

3. Explain how the use of CNG in automobiles has reduced pollution in

our cities.

4. Compare LPG and wood as fuels.

5. Give reasons.

(a) Water is not used to control fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

around an aluminium pipe does not.

6. Make a labelled diagram of a candle flame.

7. Name the unit in which the calorific value of a fuel is expressed.

8. Explain how CO

is able to control fires.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily.

Explain.

10. Which zone of a flame does a goldsmith use for melting gold and silver

and why?

11. In an experiment 4.5 kg of a fuel was completely burnt. The heat produced

was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

12. Can the process of rusting be called combustion? Discuss.

13. Abida and Ramesh were doing an experiment in which water was to be

heated in a beaker. Abida kept the beaker near the wick in the yellow part

of the candle flame. Ramesh kept the beaker in the outermost part of the

flame. Whose water will get heated in a shorter time?

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baking soda +

vinegar

Extended Learning — Activities and Projects

1. Survey the availability of various fuels in your locality. Find out

their cost per kg and prepare a tabular chart showing how many

kJ of various fuels you can get for every rupee.

2. Find out the number, type and location of fire extinguishers available

in your school, nearby shops and factories. Write a brief report about

the preparedness of these establishments to fight fire.

3. Survey 100 houses in your area. Find the percentage of households

using LPG, kerosene, wood and cattle dung as fuel.

4. Talk to people who use LPG at home. Find out what precautions

they take in using LPG.

5. Make a model of a fire extinguisher. Place a short candle and a

slightly taller candle in a small dish filled with baking soda. Place

the dish at the bottom of a large bowl. Light both the candles. Then

pour vinegar into the dish of baking soda. Take care. Do not pour

vinegar on the candles. Observe the foaming reaction. What happens

to the candles? Why? In what order?

Fig. 4.15

For more information, visit:

l www.newton.dep.anl.gov/askasci/chem03/chem03767.htm

l http://en.wikipedia.org/wiki/combustion

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